This invention relates generally to improvements in prosthetic devices used for reconstruction of the knee joint in humans. More particularly, this invention relates to an improved knee prosthesis, wherein the posterior stabilization of the prosthesis is enhanced during flexion movements, and further wherein flexibility is provided to the surgeon during the implantation as to the level of stabilization that is to be utilized.
Artificial or prosthetic joint mechanisms for implantation into animals, particularly humans, have been the subject of intensive research and development efforts for many years. Such prosthetic joint mechanisms have typically comprised one or more implant components formed from a relatively biostable material having selected structural properties and a unique shape to replace all or part of a selected anatomical joint, for example, a hip or knee joint. The implant components are installed by surgically accessing the joint and by resection of one or more bone surfaces to accommodate direct attachment thereto of the implant components. In the past, attachment of implant components to patient bone has been commonly achieved by use of bone cement, such as a methyl methacrylate-based cement or the like used as a grouting material to fill up the space between the receptive bone surface and the prosthetic component. More recently, however, a variety of structural and biological imcompatibility problems encountered with the use f bone cements have led to the development of so-called bone ingrowth materials. In such bone ingrowth materials, a surface coating of controlled porosity is provided on a prosthesis component in a position for intimately contacting patient bone to achieve a significant degree of postoperative bone and/or tissue ingrowth, and thereby obtain a mechanical interlock with patient bone without utilizing bone cement.
The human knee joint has presented particularly difficult problems in the development of a satisfactory prosthetic joint. More specifically, the human knee joint is recognized as an extremely complex mechanical structure which is subjected to high mechanical loads of widely varying magnitude and direction during normal function. Unfortunately, the knee joint is also subject to a relatively high frequency of disabling injury occurrence since the knee joint bears full body weight and can be exposed to twisting forces and blows from all directions. The greatest risk lies in sports that include pivoting, twisting, running, and jumping, such as tennis, basketball, skiing and racquetball, and, of course, contact sports such as soccer and football.
As a result, a wide variety of knee prostheses have been proposed in the prior art, typically to include matingly configured femoral and tibial components adapted respectively for implantation onto the lower end of a resected femur and the upper end of a resected tibia, with appropriate plastic meniscal bearing components interposed therebetween. In the majority of these prior art knee prostheses, the general configuration of the femoral and tibial components has resembled the general physiology of the natural knee joint, namely, to include medial and lateral condyles on the femoral component which are supported by the meniscal bearing components on the tibial component. Desirably, the prosthesis components are retained in bearing engagement throughout a range of knee flexion by the natural connective tissues including the ligament and tendon structure of the knee joint.
In some cases involving significant damage or injury to the connective tissues in addition to the bone structure, it is necessary or desirable to utilize a special prosthesis device having posterior stabilization features to prevent dislocation of the prosthetic components during moderate to severe knee flexion. That is, depending upon the condition of the connective tissues, the wide range of natural knee flexion can create a significant risk of dislocation when substantial flexion angles are encountered. To address this problem, specialized prostheses having structural components designed to prevent posterior dislocation have been developed, as described, for example, in U.S. Pat. No. 4,298,992. However, since the need for posterior stabilization in a particular patient often cannot be determined until the time of surgery, it has been necessary for the surgeon to speculate regarding the patient's need for posterior stabilization or otherwise have more than one prosthesis available in the operating room for selection in the course of the implantation surgery.
There exists, therefore, a significant need for an improved knee prosthesis which can be adapted quickly and easily at the time of implantation surgery to provide posterior stabilization in accordance with patient need. The present invention fulfills these needs and provides further related advantages.